The invention is generally related to electronic data storage systems which are associated with a network. Enterprise data storage systems are used to maintain relatively large data sets. A simple data storage subsystem includes a storage array and a host device. Applications Read and Write data by sending IO requests to the host device via the network. The host device translates the IO requests from a format characterized by designators used by applications, e.g., filename, into a format characterized by designators used by the storage array, e.g., LUN, cylinder and track, in order to prompt a Read or Write in accordance with the IO request. Clusters of hosts and storage arrays can be organized into a data center. Groups of data centers can be organized into a single data storage system.
Large data storage systems are relatively costly and complex. The performance of a data storage system is determined in part by the performance of individual devices. The performance of a storage array is a function of configuration details including the types of physical storage devices with which it is equipped. Typically, slower storage devices are less costly than faster devices so there is a tradeoff between storage capacity and performance. The performance of a host is dependent on such variables as processor performance and loading so increased performance is generally associated with increased cost. Host performance is also dependent on the underlying performance of the storage. Storage system performance is also affected by network performance, with higher performance links being more costly. Because of the relationship between performance and cost it is common practice to match storage system performance to anticipated performance requirements so as to avoid purchase of unnecessary storage resources. However, this is a complex task because different applications have different performance and capacity requirements, and those requirements may change over time. Further complicating matters, the costs associated with an additional unit of storage capacity are variable. For example, there may be no additional cost for an enterprise to utilize an additional unit of unused storage capacity, but adding a new unit of storage capacity may require upgrade of devices or even purchase of additional devices.